In the past, measurements of propagation velocities of elastic waves in boreholes have been carried out in order to determine a characteristic of the formations down hole. Such measurements are of interest to geophysicists and geologists. Various logging tools are known and have been used to measure the propagation velocity of compressional waves (P-waves) in the formation surrounding the borehole. Also, more recently, shear wave seismics has made measurement of the propagation velocity of that type of waves an important objective.
The various logging tools which have been employed heretofore have made direct measurements of the velocity of compressional waves. For example, a logging sonde would have a source of acoustic energy transmit a short signal which propagated within a bore hole and in the surrounding rock. That signal would reach two spaced receivers along several paths. Since the path to reach the furthest one was longer by the distance between them, the propagation velocity was related to the arrival time difference. In practice, the meaurement was modified in several ways to correct for hole irregularities etc. However, sophisticated circuitry was required to reliably detect the first arrival of the signals transmitted by the source. Furthermore, event stretch or in extreme cases cycle skipping have been among the difficulties encountered which could lead to erroneous measurements.
Another difficulty has been the problem of detecting later arrivals. Shear wave velocities are lower than pressure or P-wave velocities, and the superposition of several other later arriving waves, e.g. mud waves and Rayleigh waves makes shear wave arrivals difficult to detect.
Consequently, it is an object of this invention to teach a method and apparatus for measuring the wave length of arriving signals and from such measurement to determine the propagation velocity thereof.